Golf ball

ABSTRACT

A core  4  of a golf ball  2  includes a center  10  and an envelope layer  12  positioned outside the center  10 . The ratio of the volume of the core  4  to the volume of a phantom sphere of the golf ball  2  is equal to or greater than 78%. The difference (He−Ho) between a JIS-C hardness He at the surface of the core  4  and a JIS-C hardness Ho at the central point of the core  4  is equal to or greater than 15 but equal to or less than 40. The difference (H 1 −Ho) between a JIS-C hardness H 1  at a point P 1  that is located radially outward of the boundary between the center  10  and the envelope layer  12  and whose distance from the boundary is 1 mm and the JIS-C hardness Ho is equal to or greater than 0 but equal to or less than 2.

This application claims priority on Patent Application No. 2011-92211filed in JAPAN on Apr. 18, 2011. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the presentinvention relates to multi-piece golf balls that include a center, anenvelope layer, a mid layer, and a cover.

2. Description of the Related Art

Golf players'foremost requirement for golf balls is flight performance.Golf players place importance on flight performance upon shots with adriver, a long iron, and a middle iron. Flight performance correlateswith the resilience performance of a golf ball. When a golf ball havingexcellent resilience performance is hit, the golf ball flies at a highspeed, thereby achieving a large flight distance.

An appropriate trajectory height is required in order to achieve a largeflight distance. A trajectory height depends on a spin rate and a launchangle. In a golf ball that achieves a high trajectory by a high spinrate, a flight distance is insufficient. In a golf ball that achieves ahigh trajectory by a high launch angle, a large flight distance isobtained. Use of a core having an outer-hard/inner-soft structure canachieve a low spin rate and a high launch angle.

Golf players also place importance on spin performance of golf balls.When a backspin rate is high, the run is short. It is easy for golfplayers to cause a golf ball, to which backspin is easily provided, tostop at a target point. When a sidespin rate is high, the golf balltends to curve. It is easy for golf players to intentionally cause agolf ball, to which sidespin is easily provided, to curve. A golf ballto which spin is easily provided has excellent controllability. Inparticular, advanced golf players place importance on controllabilityupon a shot with a short iron.

In light of achieving various performance characteristics, golf ballseach having a multilayer structure have been proposed. JPH9-56848/JP No.2888197 (U.S. Pat. No. 5,725,442), JPH8-336618/JP No. 2817668 (U.S. Pat.No. 5,733,205), and JPH10-328326/JP No. 3985107 (U.S. Pat. No.6,468,169) each disclose a golf ball that includes a core, an envelopelayer, an inner cover, and an outer cover. JP2004-130072/JP No. 4214003(US2004/0029648) discloses a golf ball that includes a core and a cover.The core has a three-layer structure. The principal component of thecover is a polyurethane thermoplastic elastomer. JP2001-17575/JP No.3525813 (U.S. Pat. No. 6,271,296) discloses a golf ball that includes acore, an envelope layer, a mid layer, and a cover.

In a golf ball that includes a core consisting of a center and anenvelope layer, a large and hard center may be used. Spin is easilyprovided to this golf ball. In addition, the energy loss is high, andthus the resilience performance deteriorates. Therefore, even when thisgolf ball is hit with a long iron or a middle iron, a sufficient flightdistance is not obtained.

An object of the present invention is to provide a golf ball thatprovides a large flight distance when being hit with a long iron or amiddle iron.

SUMMARY OF THE INVENTION

A golf ball according to the present invention comprises a core, a midlayer positioned outside the core, and a cover positioned outside themid layer. The core comprises a center and an envelope layer positionedoutside the center. A ratio of a volume of the core to a volume of aphantom sphere of the golf ball is equal to or greater than 78%. Adifference (He−Ho) between a JIS-C hardness He at a surface of the coreand a JIS-C hardness Ho at a central point of the core is equal to orgreater than 15 but equal to or less than 40. A difference (H1−Ho)between a JIS-C hardness H1 at a point P1 that is located radiallyoutward of a boundary between the center and the envelope layer andwhose distance from the boundary is 1 mm and the JIS-C hardness Ho atthe central point of the core is equal to or greater than 0 but equal toor less than 2.

Preferably, in the golf ball, a JIS-C hardness Hc of the cover is lessthan 65.

Preferably, in the golf ball, a thickness of the cover is less than 0.8mm.

Preferably, in the golf ball, a JIS-C hardness Hm of the mid layer isequal to or greater than 90.

Preferably, in the golf ball, a thickness of the mid layer is equal toor less than 1.5 mm.

Preferably, in the golf ball, a JIS-C hardness Hc of the cover is lessthan the JIS-C hardness Ho at the central point of the core.

Preferably, in the golf ball, a JIS-C hardness Hm of the mid layer isgreater than the JIS-C hardness He at the surface of the core.

Preferably, in the golf ball, the cover is formed from a resincomposition. A shear loss elastic modulus G″ of the resin composition,which is measured under conditions of a vibration frequency of 10 Hz anda temperature of 0° C., is equal to or less than 1.95×10⁷ Pa. A ratio(E″/G″) of a tensile loss elastic modulus E″ of the resin composition,which is measured under conditions of a vibration frequency of 10 Hz anda temperature of 0° C., to the shear loss elastic modulus G″ is equal toor greater than 1.76.

Preferably, in the golf ball, a principal component of a base materialof the resin composition is a thermoplastic polyurethane. A polyolcomponent of the thermoplastic polyurethane is polytetramethylene etherglycol having a number average molecular weight of 1500 or less.

Preferably, in the golf ball, the JIS-C hardness Ho at the central pointof the core is equal to or greater than 40 but equal to or less than 80.

Preferably, in the golf ball, a diameter of the center is equal to orgreater than 10 mm but equal to or less than 20 mm.

Preferably, in the golf ball, the envelope layer is obtained by a rubbercomposition being crosslinked.

Preferably, in the golf ball, a JIS-C hardness at a surface of theenvelope layer is equal to or greater than 75 but equal to or less than95.

Preferably, in the golf ball, a thickness of the envelope layer is equalto or greater than 8 mm but equal to or less than 18 mm.

Preferably, in the golf ball, a difference (Ho−Hc) between a JIS-Chardness Hc of the cover and the JIS-C hardness Ho at the central pointof the core is equal to or greater than 3 but equal to or less than 40.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ballaccording to one embodiment of the present invention;

FIG. 2 is a graph showing a hardness distribution of a core of the golfball in FIG. 1;

FIG. 3 is a graph showing a hardness distribution of a core of a golfball according to Example 2 of the present invention;

FIG. 4 is a graph showing a hardness distribution of a core of a golfball according to Example 3 of the present invention;

FIG. 5 is a graph showing a hardness distribution of a core of a golfball according to Example 6 of the present invention;

FIG. 6 is a graph showing a hardness distribution of a core of a golfball according to Example 7 of the present invention;

FIG. 7 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 1;

FIG. 8 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 2;

FIG. 9 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 3;

FIG. 10 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 4;

FIG. 11 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 5;

FIG. 12 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 6; and

FIG. 13 is a graph showing a hardness distribution of a core of a golfball according to Comparative Example 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention, based onpreferred embodiments with reference to the accompanying drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6positioned outside the core 4, and a cover 8 positioned outside the midlayer 6. The core 4 includes a spherical center 10 and an envelope layer12 positioned outside the center 10. On the surface of the cover 8, alarge number of dimples 14 are formed. Of the surface of the golf ball2, a part other than the dimples 14 is a land 16. The golf ball 2includes a paint layer and a mark layer on the external side of thecover 8 although these layers are not shown in the drawing.

The golf ball 2 has a diameter, of 40 mm or greater but 45 mm or less.From the standpoint of conformity to the rules established by the UnitedStates Golf Association (USGA), the diameter is preferably equal to orgreater than 42.67 mm. In light of suppression of air resistance, thediameter is preferably equal to or less than 44 mm and more preferablyequal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g orgreater but 50 g or less. In light of attainment of great inertia, theweight is preferably equal to or greater than 44 g and more preferablyequal to or greater than 45.00 g. From the standpoint of conformity tothe rules established by the USGA, the weight is preferably equal to orless than 45.93 g.

The center 10 is obtained by crosslinking a rubber composition. Examplesof preferable base rubbers for use in the rubber composition includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers, and natural rubbers. In light ofresilience performance, polybutadienes are preferred. When apolybutadiene and another rubber are used in combination, it ispreferred if the polybutadiene is included as a principal component.Specifically, the proportion of the polybutadiene to the entire baserubber is preferably equal to or greater than 50% by weight and morepreferably equal to or greater than 80% by weight. The proportion ofcis-1,4 bonds in the polybutadiene is preferably equal to or greaterthan 40% and more preferably equal to or greater than 80%.

The rubber composition of the center 10 includes an α,β-unsaturatedcarboxylic acid having 2 to 8 carbon atoms and a metal oxide. In thegolf ball 2, they both react with each other in the rubber composition,whereby a salt is obtained. The salt serves as a co-crosslinking agent.Examples of preferable α,β-unsaturated carboxylic acids include acrylicacid and methacrylic acid. Examples of preferable metal oxides includezinc oxide and magnesium oxide. From the standpoint that processabilityis good, a combination of methacrylic acid and magnesium oxide is morepreferred. The amount of the α,β-unsaturated carboxylic acid ispreferably equal to or greater than 15 parts by weight but equal to orless than 45 parts by weight, per 100 parts by weight of the baserubber. From the standpoint that the center 10 is appropriatelycrosslinked and can contribute to resilience performance, the amount ofthe α,β-unsaturated carboxylic acid is particularly preferably equal toor greater than 20 parts by weight per 100 parts by weight of the baserubber. From the standpoint that the flexibility of the center 10 isappropriately maintained and the golf ball 2 having excellent feel atimpact is obtained, the amount of the α,β-unsaturated carboxylic acid isparticularly preferably equal to or less than 40 parts by weight per 100parts by weight of the base rubber. The amount of the metal oxide ispreferably equal to or greater than 20 parts by weight but equal to orless than 50 parts by weight, per 100 parts by weight of the baserubber. From the standpoint that the center 10 is appropriatelycrosslinked and can contribute to resilience performance, the amount ofthe metal oxide is particularly preferably equal to or greater than 25parts by weight per 100 parts by weight of the base rubber. From thestandpoint that the flexibility of the center 10 is appropriatelymaintained and the golf ball 2 having excellent feel at impact isobtained, the amount of the metal oxide is particularly preferably equalto or less than 45 parts by weight per 100 parts by weight of the baserubber.

A monovalent or bivalent metal salt of an α,β-unsaturated carboxylicacid having 2 to 8 carbon atoms may be included as a co-crosslinkingagent in the rubber composition of the center 10. In this case, theamount of the co-crosslinking agent is preferably equal to or greaterthan 5 parts by weight but equal to or less, than 30 parts by weight,per 100 parts by weight of the base rubber. From the standpoint that thecenter 10 is appropriately crosslinked and can contribute to resilienceperformance, the amount is more preferably equal to or greater than 10parts by weight per 100 parts by weight of the base rubber. From thestandpoint that the flexibility of the center 10 is appropriatelymaintained and the golf ball 2 having excellent feel at impact isobtained, the amount is more preferably equal to or less than 25 partsby weight and particularly preferably equal to or less than 20 parts byweight per 100 parts by weight of the base rubber. Specific examples ofthe monovalent or bivalent metal salt of the α,β-unsaturated carboxylicacid having 2 to 8 carbon atoms include zinc acrylate, magnesiumacrylate, zinc methacrylate, and magnesium methacrylate. From thestandpoint that processability is good, magnesium methacrylate and zincmethacrylate are preferred.

Preferably, the rubber composition of the center 10 includes an organicperoxide together with an α,β-unsaturated carboxylic acid having 2 to 8carbon atoms and a metal oxide. The organic peroxide serves as acrosslinking initiator. The organic peroxide contributes to theresilience performance of the golf ball 2. Examples of suitable organicperoxides include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy) hexane, and di-t-butyl peroxide. In light ofversatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of theorganic peroxide is preferably equal to or greater than 0.1 parts byweight, more preferably equal to or greater than 0.2 parts by weight,and particularly preferably equal to or greater than 0.3 parts byweight, per 100 parts by weight of the base rubber. In light of softfeel at impact, the amount of the organic peroxide is preferably equalto or less than 1.5 parts by weight, more preferably equal to or lessthan 1.0 parts by weight, and particularly preferably equal to or lessthan 0.8 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the center 10. Examples of suitable fillers include zincoxide, barium sulfate, calcium carbonate, and magnesium carbonate. Theamount of the filler is determined as appropriate so that the intendedspecific gravity of the center 10 is accomplished. A particularlypreferable filler is zinc oxide. Zinc oxide serves not only as aspecific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer,a dispersant, sulfur, a vulcanization accelerator, and the like areadded to the rubber composition of the center 10. Crosslinked rubberpowder or synthetic resin powder may also be dispersed in the rubbercomposition. The center 10 may include an organic sulfur compounddescribed later.

During formation of the center 10, the rubber composition of the center10 is placed into a mold including upper and lower mold halves eachhaving a hemispherical cavity, and is compressed and heated. By thisheating, a crosslinking reaction takes place to complete the center 10.The crosslinking temperature is generally equal to or higher than 140°C. but equal to or lower than 180° C. The time period for crosslinkingthe center 10 is generally equal to or longer than 10 minutes but equalto or shorter than 60 minutes.

In light of durability, a hardness Ho at the central point of the center10 (i.e., the central point of the core 4) is preferably equal to orgreater than 40, more preferably equal to or greater than 45, andparticularly preferably equal to or greater than 50. In light ofsuppression of spin, the central hardness Ho is preferably equal to orless than 80, more preferably equal to or less than 75, and particularlypreferably equal to or less than 70. The central hardness Ho is measuredby pressing a JIS-C type hardness scale against the central point of acut plane of the center 10 that has been cut into two halves. For themeasurement, an automated rubber hardness measurement machine (tradename “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which thishardness scale is mounted, is used.

In the golf ball 2, the center 10 has a diameter of preferably 10 mm orgreater but 20 mm or less. The center 10 having a diameter of 10 mm orgreater can achieve excellent feel at impact.

In this respect, the diameter is more preferably equal to or greaterthan 12 mm and particularly preferably equal to or greater than 13 mm.When the center 10 has a diameter of 20 mm or less, the envelope layer12 having a sufficiently large thickness can be formed. In this respect,the diameter is more preferably equal to or less than 18 mm andparticularly preferably equal to or less than 17 mm.

The envelope layer 12 is obtained by crosslinking a rubber composition.Examples of preferable base rubbers for use in the rubber compositioninclude polybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers, and natural rubbers. In light ofresilience performance, polybutadienes are preferred. When apolybutadiene and another rubber are used in combination, it ispreferred if the polybutadiene is included as a principal component.Specifically, the proportion of the polybutadiene to the entire baserubber is preferably equal to or greater than 50% by weight and morepreferably equal to or greater than 80% by weight. The proportion ofcis-1,4 bonds in the polybutadiene is preferably equal to or greaterthan 40% and more preferably equal to or greater than 80%.

In order to crosslink the envelope layer 12, a co-crosslinking agent ispreferably used. Examples of preferable co-crosslinking agents in lightof resilience performance include monovalent or bivalent metal salts ofan α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specificexamples of preferable co-crosslinking agents include zinc acrylate,magnesium acrylate, zinc methacrylate, and magnesium methacrylate. Inlight of resilience performance and feel at impact, zinc acrylate andzinc methacrylate are particularly preferred. It should be noted thatinstead of the co-crosslinking agent, the aforementioned α,β-unsaturatedcarboxylic acid having 2 to 8 and the aforementioned metal oxide may beincluded in the rubber composition of the envelope layer 12.

In the golf ball 2, the amount of the co-crosslinking agent ispreferably equal to or greater than 20 parts by weight but equal to orless than 50 parts by weight per 100 parts by weight of the base rubber.From the standpoint that the envelope layer 12 is appropriatelycrosslinked and can contribute to resilience performance, the amount ismore preferably equal to or greater than 25 parts by weight andparticularly preferably equal to or greater than 30 parts by weight per100 parts by weight of the base rubber. From the standpoint that theflexibility of the envelope layer 12 is appropriately maintained and thegolf ball 2 having excellent feel at impact is obtained, the amount ismore preferably equal to or less than 45 parts by weight andparticularly preferably equal to or less than 40 parts by weight per 100parts by weight of the base rubber.

Preferably, the rubber composition of the envelope layer 12 includes anorganic peroxide together with a co-crosslinking agent. The organicperoxide serves as a crosslinking initiator. The organic peroxidecontributes to the resilience performance of the golf ball 2. Examplesof suitable organic peroxides include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of theorganic peroxide is preferably equal to or greater than 0.1 parts byweight, more preferably equal to or greater than 0.3 parts by weight,and particularly preferably equal to or greater than 0.5 parts byweight, per 100 parts by weight of the base rubber. In light of softfeel at impact, the amount of the organic peroxide is preferably equalto or less than 2.0 parts by weight, more preferably equal to or lessthan 1.5 parts by weight, and particularly preferably equal to or lessthan 1.0 parts by weight, per 100 parts by weight of the base rubber.

The rubber composition of the envelope layer 12 may include an organicsulfur compound. Examples of preferable organic sulfur compounds includemonosubstitutions such as diphenyl disulfide,bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide,bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide,bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide,bis(4-cyanophenyl)disulfide, and the like; disubstitutions such asbis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide,bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide,bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide,bis(2-cyano-5-bromophenyl) disulfide, and the like; trisubstitutionssuch as bis(2,4,6-trichlorophenyl) disulfide,bis(2-cyano-4-chloro-6-bromophenyl) disulfide, and the like;tetrasubstitutions such as bis(2,3,5,6-tetrachlorophenyl) disulfide andthe like; and pentasubstitutions such asbis(2,3,4,5,6-pentachlorophenyl) disulfide,bis(2,3,4,5,6-pentabromophenyl) disulfide, and the like. The organicsulfur compound contributes to resilience performance. More preferableorganic sulfur compounds are diphenyl disulfide andbis(pentabromophenyl) disulfide. A particularly preferable organicsulfur compound is bis(pentabromophenyl) disulfide.

In light of resilience performance of the golf ball 2, the amount of theorganic sulfur compound is preferably equal to or greater than 0.1 partsby weight and more preferably equal to or greater than 0.2 parts byweight per 100 parts by weight of the base rubber. In light of soft feelat impact, the amount of the organic sulfur compound is preferably equalto or less than 1.5 parts by weight, more preferably equal to or lessthan 1.0 parts by weight, and particularly preferably equal to or lessthan 0.8 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the envelope layer 12. Examples of suitable fillersinclude zinc oxide, barium sulfate, calcium carbonate, and magnesiumcarbonate. Powder of a metal with a high specific gravity may beincluded as a filler. Specific examples of metals with a high specificgravity include tungsten and molybdenum. The amount of the filler isdetermined as appropriate so that the intended specific gravity of theenvelope layer 12 is accomplished. A particularly preferable filler iszinc oxide. Zinc oxide serves not only as a specific gravity adjusterbut also as a crosslinking activator. According to need, variousadditives such as sulfur, an anti-aging agent, a coloring agent, aplasticizer, a dispersant, and the like are included in the envelopelayer 12 in an adequate amount. Crosslinked rubber powder or syntheticresin powder may also be included in the envelope layer 12.

During formation of the envelope layer 12, the center 10 is covered withtwo uncrosslinked or semi-crosslinked half shells. These half shells arecompressed and heated. By this heating, a crosslinking reaction takesplace to complete the envelope layer 12. The crosslinking temperature isgenerally equal to or higher than 140° C. but equal to or lower than180° C. The time period for crosslinking the envelope layer 12 isgenerally equal to or longer than 10 minutes but equal to or shorterthan 60 minutes.

The hardness of the envelope layer 12 gradually increases from itsinnermost portion toward its surface. In light of resilienceperformance, a hardness He at the surface of the envelope layer 12(i.e., the surface of the core 4) is preferably equal to or greater than75, more preferably equal to or greater than 80, and particularlypreferably equal to or greater than 85. In light of feel at impact, thehardness He is preferably equal to or less than 95, more preferablyequal to or less than 93, and particularly preferably equal to or lessthan 92. The hardness He is measured by pressing a JIS-C type hardnessscale against the surface of the core 4. For the measurement, anautomated rubber hardness measurement machine (trade name “P1”,manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scaleis mounted, is used.

In the golf ball 2, the envelope layer 12 has a thickness of preferably8 mm or greater but 18 mm or less. The envelope layer 12 having athickness of 8 mm or greater can suppress spin. In this respect, thethickness is more preferably equal to or greater than 9 mm andparticularly preferably equal to or greater than 10 mm. When theenvelope layer 12 has a thickness of 18 mm or less, the center 10 havinga large diameter can be formed. The center 10 having a large diametercan contribute to soft feel at impact. In this respect, the thickness ismore preferably equal to or less than 16 mm and particularly preferablyequal to or less than 15 mm.

FIG. 2 shows a hardness distribution of the core 4. The vertical axisindicates a JIS-C hardness. The horizontal axis indicates a distancefrom the central point Po of the core 4. A hardness at the central pointPo is the aforementioned central hardness Ho. A chain double-dashed lineLB indicates the position of the boundary between the center 10 and theenvelope layer 12 in the core 4. A point P1 indicates a position, on thecore 4, which is located radially outward of the boundary LB between thecenter 10 and the envelope layer 12 and whose radial distance from theboundary LB is 1 mm. In FIG. 2, a hardness at the point P1 is indicatedas H1. The hardness H1 corresponds to a hardness at the innermostportion of the envelope layer 12. A point Pe indicates the position ofthe surface of the core 4. A hardness at the point Pe is theaforementioned surface hardness He. Hardnesses other than the surfacehardness He are measured by pressing a JIS-C type hardness scale againsta cut plane of the core 4 that has been cut into two halves. For themeasurement, an automated rubber hardness measurement machine (tradename “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which thishardness scale is mounted, is used.

In FIG. 2, the portion from the central point Po to the boundary LBindicates a hardness distribution of the center 10. The portion from theboundary LB to the surface Pe indicates a hardness distribution of theenvelope layer 12. As shown in FIG. 2, the hardness of the center 10 isless than the hardness of the envelope layer 12 as a whole. In the golfball 2, the center 10 is flexible, and the envelope layer 12 is hard.The core 4 has an outer-hard/inner-soft structure. The core 4 cancontribute to feel at impact. It is difficult to provide spin to thegolf ball 2 that includes the core 4. When the golf ball 2 is hit with adriver, the flight distance is large. When the golf ball 2 is hit with along iron or a middle iron, the flight distance is also large.

In the golf ball 2, the hardness of the center 10 shifts from thecentral point Po toward the surface with nearly equal values. Thevariation of the hardness of the center 10 is small. The center 10 caneffectively suppress deterioration of resilience performance. When thegolf ball 2 is hit with a driver, the flight distance is large. When thegolf ball 2 is hit with a long iron or a middle iron, the flightdistance is also large.

In the golf ball 2, the surface hardness He of the envelope layer 12 isgreater than the hardness at the innermost portion of the envelope layer12. The envelope layer 12 can contribute to decrease of a spin rate.When the golf ball 2 is hit with a driver, the flight distance is large.When the golf ball 2 is hit with a long iron or a middle iron, theflight distance is also large.

In the golf ball 2, from the standpoint that deterioration of resilienceperformance can be prevented, the difference (He−Ho) between the surfacehardness He and the central hardness Ho is preferably equal to or lessthan 40 and more preferably equal to or less than 35. In light ofsuppression of spin, the difference (He−Ho) is preferably equal to orgreater than 15.

As shown in FIG. 2, at the boundary LB, the hardness of the center 10and the hardness of the envelope layer 12 are equalized with each other.The hardness of the core 4 is not unique at the boundary LB. The core 4can contribute to achievement of an outer-hard/inner-soft structurehaving continuity throughout the golf ball 2. The core 4 has anappropriate hardness distribution. The core 4 can contribute to decreaseof a spin rate. The core 4 can suppress deterioration of resilienceperformance. When the golf ball 2 is hit with a driver, the flightdistance is large. When the golf ball 2 is hit with a long iron or amiddle iron, the flight distance is also large.

In the golf ball 2, the hardness H1 at the point P1 is slightly greaterthan or equal to the central hardness Ho. Specifically, the difference(H1−Ho) between the hardness H1 at the point P1 and the central hardnessHo is equal to or greater than 0 but equal to or less than 2. In thegolf ball 2, the center 10 of the core 4 has uniform hardness. The core4 can suppress deterioration of resilience performance. In addition, atthe boundary LB, the hardness of the core 4 is not unique. The core 4can contribute to achievement of an outer-hard/inner-soft structurehaving continuity throughout the golf ball 2. It is difficult to providespin to the golf ball 2 that includes the core 4. When the golf ball 2is hit with a driver, the flight distance is large. When the golf ball 2is hit with a long iron or a middle iron, the flight distance is alsolarge.

The ratio of the volume of the core 4 to the volume of a phantom sphereof the golf ball 2 is equal to or greater than 78%. In other words, thecore 4 is large. The core 4 can achieve excellent resilience performanceof the golf ball 2. The core 4 can suppress spin of the golf ball 2. Inthese respects, the ratio is more preferably equal to or greater than79% and particularly preferably equal to or greater than 80%. Thesurface of the phantom sphere is the surface of the golf ball 2 when itis postulated that no dimple 14 exists.

A resin composition is suitably used for the mid layer 6. Examples ofthe base polymer of the resin composition include ionomer resins,styrene block-containing thermoplastic elastomers, thermoplasticpolyester elastomers, thermoplastic polyamide elastomers, andthermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. Ionomer resinsare highly elastic. As described later, the cover 8 of the golf ball 2is thin and flexible. Thus, when the golf ball 2 is hit with a driver,the mid layer 6 significantly deforms. The mid layer 6 including anionomer resin contributes to resilience performance upon a shot with adriver. An ionomer resin and another resin may be used in combination.In this case, in light of resilience performance, the proportion of theionomer resin to the entire base polymer is preferably equal to orgreater than 50% by weight, more preferably equal to or greater than 70%by weight, and particularly preferably equal to or greater than 85% byweight.

Examples of preferable ionomer resins include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. A preferable binary copolymer includes 80% by weight ormore but 90% by weight or less of an α-olefin, and 10% by weight or morebut 20% by weight or less of an α,β-unsaturated carboxylic acid. Thebinary copolymer has excellent resilience performance. Examples of otherpreferable ionomer resins include ternary copolymers formed with: anα-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms;and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. Apreferable ternary copolymer includes 70% by weight or more but 85% byweight or less of an α-olefin, 5% by weight or more but 30% by weight orless of an α,β-unsaturated carboxylic acid, and 1% by weight or more but25% by weight or less of an α,β-unsaturated carboxylate ester. Theternary copolymer has excellent resilience performance. For the binarycopolymer and the ternary copolymer, preferable α-olefins are ethyleneand propylene, while preferable α,β-unsaturated carboxylic acids areacrylic acid and methacrylic acid. A particularly preferable ionomerresin is a copolymer formed with ethylene and acrylic acid ormethacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxylgroups are neutralized with metal ions. Examples of metal ions for usein neutralization include sodium ion, potassium ion, lithium ion, zincion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. Theneutralization may be carried out with two or more types of metal ions.Particularly suitable metal ions in light of resilience performance anddurability of the golf ball 2 are sodium ion, zinc ion, lithium ion, andmagnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”,“Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan1856”, “Himilan 1855”, “Himilan AM7311”, “HimilanAM7315”,“HimilanAM7317”, “HimilanAM7318”, “Himilan AM7329”, “Himilan MK7320”,and “Himilan MK7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co.,Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”,“Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “SurlynAD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont deNemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured byExxonMobil Chemical Corporation.

Two or more ionomer resins may be used in combination for the mid layer6. An ionomer resin neutralized with a monovalent metal ion, and anionomer resin neutralized with a bivalent metal ion may be used incombination.

The mid layer 6 may include a highly elastic resin. Examples of highlyelastic resins include polybutylene terephthalate, polyphenylene ether,polyethylene terephthalate, polysulfone, polyethersulfone, polyphenylenesulfide, polyarylate, polyamide imide, polyetherimide, polyether etherketone, polyimide, polytetrafluoroethylene, polyamino bismaleimide,polybisamide triazole, polyphenylene oxide, polyacetal, polycarbonate,acrylonitrile-butadiene-styrene copolymers, and acrylonitrile-styrenecopolymers.

According to need, a coloring agent such as titanium dioxide, a fillersuch as barium sulfate, a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, a fluorescent material, a fluorescentbrightener, and the like are included in the mid layer 6 in an adequateamount. For forming the mid layer 6, known methods such as injectionmolding, compression molding, and the like can be used.

The mid layer 6 has a hardness Hm of preferably 90 or greater. The midlayer 6 having a hardness Hm of 90 or greater achieves excellentresilience performance of the golf ball 2. The mid layer 6 having ahardness Hm of 90 or greater can achieve an outer-hard/inner-softstructure of the sphere consisting of the core 4 and the mid layer 6.The sphere having the outer-hard/inner-soft structure suppresses spin ofthe golf ball 2. In these respects, the hardness Hm is particularlypreferably equal to or greater than 92. In light of durability and feelat impact, the hardness Hm is preferably equal to or less than 98 andparticularly preferably equal to or less than 97. From the standpointthat an outer-hard/inner-soft structure having continuity throughout thegolf ball 2 is achieved and spin is suppressed, preferably, the hardnessHm of the mid layer 6 is greater than the surface hardness He of thecore 4, and the surface hardness He of the core 4 is greater than thesurface hardness of the center 10.

The hardness Hm is measured with a JIS-C type spring hardness scalemounted to an automated rubber hardness measurement machine (trade name“P1”, manufactured by Kobunshi Keiki Co., Ltd.). For the measurement, aslab formed by hot press and having a thickness of about 2 mm is used. Aslab kept at 23° C. for two weeks is used for the measurement. At themeasurement, three slabs are stacked. A slab formed from a resincomposition that is the same as the resin composition of the mid layer 6is used for the measurement.

In light of suppression of spin, the mid layer 6 has a thickness ofpreferably 0.3 mm or greater and particularly preferably 0.5 mm orgreater. In light of feel at impact, the thickness is preferably equalto or less than 1.5 mm, more preferably equal to or less than 1.2 mm,and particularly preferably equal to or less than 1.0 mm.

The cover 8 is formed from a resin composition. Examples of the basepolymer of the resin composition include polyurethanes, polyesters,polyamides, polyolefins, polystyrenes, and ionomer resins. Particularly,polyurethanes are preferred. Polyurethanes are flexible. When the golfball 2 that includes the cover 8 including a polyurethane is hit with ashort iron, the spin rate is high. The cover 8 formed from apolyurethane contributes to the controllability upon a shot with a shortiron. The polyurethane also contributes to the scuff resistance of thecover 8.

When the golf ball 2 is hit with a driver, a long iron, or a middleiron, the sphere consisting of the core 4 and the mid layer 6 becomessignificantly distorted since the head speed is high. Since this spherehas an outer-hard/inner-soft structure, the spin rate is suppressed. Thesuppression of the spin rate achieves a large flight distance. When thegolf ball 2 is hit with a short iron, this sphere becomes less distortedsince the head speed is low. When the golf ball 2 is hit with a shortiron, the behavior of the golf ball 2 mainly depends on the cover 8.Since the cover 8 including the polyurethane is flexible, a high spinrate is obtained. The high spin rate achieves excellent controllability.In the golf ball 2, both desired flight performance upon shots with adriver, a long iron, and a middle iron and desired controllability upona shot with a short iron are achieved.

When the golf ball 2 is hit, the cover 8 including the polyurethaneabsorbs the shock. This absorption achieves soft feel at impact.Particularly, when the golf ball 2 is hit with a short iron or a putter,the cover 8 achieves excellent feel at impact.

When being hit, compressive stress is applied to the cover 8 due tomovement of the head of a golf club. Since the face of the golf club hasa loft angle, shear stress is also applied to the cover 8 when beinghit. The head speed of a short iron is low, and the loft angle of ashort iron is high. Thus, when the golf ball 2 is hit with a short iron,the shear stress greatly influences the deformation behavior of thecover 8. The head speed of a driver is high, and the loft angle of adriver is low. Thus, when the golf ball 2 is hit with a driver, thecompressive stress greatly influences the deformation behavior of thecover 8.

The cover 8 has a shear loss elastic modulus G″ of preferably 1.95×10⁷Pa or less. As described above, when being hit with a short iron, thedeformation behavior of the cover 8 is greatly influenced by the shearstress. The spin rate obtained when being hit with a short ironcorrelates with the shear loss elastic modulus G″. When the golf ball 2that includes the cover 8 having a shear loss elastic modulus G″ of1.95×10⁷ Pa or less is hit with a short iron, the spin rate is high. Thecover 8 can achieve excellent controllability. In this respect, theshear loss elastic modulus G″ is particularly preferably equal to orless than 1.83×10⁷ Pa. In light of ease of forming the cover 8, theshear loss elastic modulus G″ is preferably equal to or greater than1.00×10⁶ Pa and particularly preferably equal to or greater than1.10×10⁶ Pa.

The ratio (E″/G″) of a tensile loss elastic modulus E″ of the cover 8 tothe shear loss elastic modulus G″ is preferably equal to or greater than1.76. As described above, when being hit with a driver, the deformationbehavior of the cover 8 is greatly influenced by the compressive stress.The spin rate obtained when being hit with a driver correlates with thetensile loss elastic modulus E″. When the golf ball 2 that includes thecover 8 having a ratio (E″/G″) of 1.76 or greater is hit with a driver,the spin rate is low, and when the golf ball 2 is hit with a short iron,the spin rate is high. In this respect, the ratio (E″/G″) is morepreferably equal to or greater than 1.86 and particularly preferablyequal to or greater than 1.90. In light of ease of forming the cover 8,the ratio (E″/G″) is preferably equal to or less than 6.0 andparticularly preferably equal to or less than 5.5.

The tensile loss elastic modulus E″ is preferably equal to or greaterthan 2.00×10⁷ Pa, more preferably equal to or greater than 2.20×10⁷ Pa,and particularly preferably equal to or greater than 2.40×10⁷ Pa. Thetensile loss elastic modulus E″ is preferably equal to or less than1.00×10⁸ Pa.

The shear loss elastic modulus G″ and the tensile loss elastic modulusE″ can be controlled by adjusting the molecular weight of a polyol, themolecular weight of a polyisocyanate, a ratio (NCO/OH), and the like.

For measuring the shear loss elastic modulus G″, a sheet having athickness of 2 mm is obtained by press molding from a resin compositionthat is the same as the resin composition of the cover 8. A test piecehaving a width of 10 mm and an inter-clamp distance of 10 mm is punchedout from the sheet. The shear loss elastic modulus G″ is measured forthe test piece. The measurement conditions are as follows.

Apparatus: “Rheometer ARES”, manufactured by TA instruments

Measurement mode: twisting (shearing)

Measurement temperature: 0° C.

Vibration frequency: 10 Hz

Measurement distortion: 0.1% For measuring the tensile loss elasticmodulus E″, a sheet having a thickness of 2 mm is obtained by pressmolding from a resin composition that is the same as the resincomposition of the cover 8. A test piece having a width of 4 mm and aninter-clamp distance of 20 mm is punched out from the sheet. The tensileloss elastic modulus E″ is measured for the test piece. The measurementconditions are as follows.

Apparatus: the dynamic viscoelasticity measuring apparatus“Rheogel-E4000”, manufactured by UBM

Measurement mode: pulling

Measurement temperature: 0° C.

Vibration frequency: 10 Hz

Measurement distortion: 0.1%

A time for which a golf ball and a club contact each other is severalhundred microseconds. Thus, the frequency of deformation of the golfball 2 when being hit is several thousand Hz. On average, the golf ball2 is hit at substantially normal temperature (25° C.). On the basis of ageneral time conversion rule of polyurethane, a deformation having afrequency of several thousand Hz in the environment having a temperatureof 25° C. corresponds to a deformation having a frequency of 10 Hz inthe environment having a temperature of 0° C. Thus, in the presentinvention, the shear loss elastic modulus G″ and the tensile losselastic modulus E″ are measured under the conditions of a vibrationfrequency of 10 Hz and a temperature of 0° C.

A polyurethane and another resin may be used in combination for thecover 8. In this case, in light of spin performance and feel at impact,the polyurethane is included as the principal component of the basepolymer. The proportion of the polyurethane to the entire base polymeris preferably equal to or greater than 50% by weight, more preferablyequal to or greater than 70% by weight, and particularly preferablyequal to or greater than 85% by weight.

For the cover 8, thermoplastic polyurethanes and thermosettingpolyurethanes can be used. In light of productivity, thermoplasticpolyurethanes are preferred. A thermoplastic polyurethane includes apolyurethane component as a hard segment, and a polyester component or apolyether component as a soft segment.

The polyurethane includes a polyol component. As the polyol, a polymericpolyol is preferred. Specific examples of polymeric polyols includepolyether polyols such as polyoxyethylene glycol (PEG), polyoxypropyleneglycol (PPG), and polytetramethylene ether glycol (PTMG); condensedpolyester polyolssuchaspolyethyleneadipate (PEA), polybutylene adipate(PBA), and polyhexamethylene adipate (PHMA); lactone polyester polyolssuch as poly-ε-caprolactone (PCL); polycarbonate polyols such aspolyhexamethylene carbonate; and acrylic polyols. Two or more polyolsmay be used in combination.

Particularly, polytetramethylene ether glycol is preferred. The spinrate obtained when the golf ball 2 is hit with a short iron has a highcorrelation with the content of polytetramethylene ether glycol.Meanwhile, the spin rate obtained when the golf ball 2 is hit with adriver has a low correlation with the content of polytetramethyleneether glycol. The golf ball 2 including a polyurethane that includespolytetramethylene ether glycol in an appropriate amount has bothexcellent flight performance when being hit with a driver and excellentcontrollability when being hit with a short iron.

In light of controllability, the polyol has a number average molecularweight of preferably 200 or greater, more preferably 400 or greater, andparticularly preferably 650 or greater. In light of suppression of spin,the molecular weight is preferably equal to or less than 1700 and morepreferably equal to or less than 1500.

The number average molecular weight is measured by gel permeationchromatography. The measurement conditions are as follows.

Apparatus: HLC-8120GPC (manufactured by Tosoh Corporation)

Eluant: tetrahydrofuran

Concentration: 0.2% by weight

Temperature: 40° C.

Column: TSK gel Super HM-M (manufactured by Tosoh Corporation)

Sample volume: 5 microliters

Flow rate: 0.5 milliliter/min

Reference material: polystyrene (“PStQuick Kit-H” manufactured by TosohCorporation)

The polymeric polyol component has a hydroxyl value of preferably 94 mgKOH/g or greater and particularly preferably 112 mg KOH/g or greater.The hydroxyl value is preferably equal to or less than 561 mg KOH/g andparticularly preferably equal to or less than 173 mg KOH/g.

Examples of an isocyanate component in the polyurethane include aromaticpolyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, a mixture (TDI) of 2,4-toluene diisocyanate and2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI),1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate(TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate(TMXDI), and paraphenylene diisocyanate (PPDI); alicyclicpolyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI),hydrogenated xylylene diisocyanate (H₆XDI), and isophorone diisocyanate(IPDI); and aliphatic polyisocyanates such as hexamethylene diisocyanate(HDI). Two or more polyisocyanates may be used in combination. In lightof weather resistance, TMXDI, XDI, HDI, H₆XDI, IPDI, and H₁₂MDI arepreferred, and H₁₂MDI is particularly preferred.

The polyurethane may include a chain extender as its component. Examplesof chain extenders include low-molecular-weight polyols andlow-molecular-weight polyamines.

Examples of low-molecular-weight polyols include diols, triols,tetraols, and hexaols. Specific examples of diols include ethyleneglycol, diethylene glycol, propanediol, dipropylene glycol, butanediol,neopentyl glycol, pentanediol, hexanediol, heptanediol, and octanediol.Specific examples of triols include glycerin, trimethylolpropane, andhexanetriol. Specific examples of tetraols include pentaerythritol andsorbitol. 1,4-butanediol is preferred.

Examples of low-molecular-weight polyamines include aliphaticpolyamines, monocyclic aromatic polyamines, and polycyclic aromaticpolyamines. Specific examples of aliphatic polyamines includeethylenediamine, propylenediamine, butylenediamine, andhexamethylenediamine. Specific examples of monocyclic aromaticpolyamines include phenylenediamine, toluene diamine, dimethyl toluenediamine, dimethylthio toluene diamine, and xylylenediamine.

The chain extender has a number average molecular weight of preferably30 or greater, more preferably 40 or greater, and particularly 45 orgreater. The molecular weight is preferably equal to or less than 400,more preferably equal to or less than 350, and particularly preferablyequal to or less than 200. Low-molecular-weight polyols andlow-molecular-weight polyamines that are used as chain extenders arelow-molecular-weight compounds that have almost no molecular weightdistribution. Thus, the low-molecular-weight polyols and thelow-molecular-weight polyamines can be distinguished from the polymericpolyol.

Specific examples of thermoplastic polyurethanes include trade names“Elastollan NY80A”, “ElastollanNY83A”, “Elastollan NY84A”, “ElastollanNY85A”, “Elastollan NY88A”, “Elastollan NY90A”, “Elastollan NY97A”,“Elastollan NY585”, and “Elastollan XKP016N”, manufactured by BASF JapanLtd.; and trade names “RESAMINE P4585LS” and “RESAMINE PS62490”,manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

The cover 8 may be formed from a composition including a thermoplasticpolyurethane and an isocyanate compound. During or after forming thecover 8, the polyurethane is crosslinked with the isocyanate compound.

According to need, a coloring agent such as titanium dioxide, a fillersuch as barium sulfate, a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, a fluorescent material, a fluorescentbrightener, and the like are included in the cover 8 in an adequateamount.

The cover 8 has a JIS-C hardness Hc of less than 65. Use of the flexiblecover 8 can achieve excellent controllability upon a shot with a shortiron. In light of controllability, the hardness Hc is more preferablyequal to or less than 60, even more preferably equal to or less than 55,and particularly preferably equal to or less than 50. If the hardness Hcis excessively low, the flight performance upon a shot with a driver isinsufficient. In this respect, the hardness Hc is preferably equal to orgreater than 20, more preferably equal to or greater than 25, andparticularly preferably equal to or greater than 35. For measuring thehardness Hc, a slab formed from a resin composition that is the same asthe resin composition of the cover 8 is used. The measurement method isthe same as the measurement method for the hardness Hm of the mid layer6.

The hardness Hc of the cover 8 is less than the central hardness Ho ofthe core 4. The golf ball 2 has excellent controllability upon a shotwith a short iron. In light of controllability, the difference (Ho−Hc)is preferably equal to or greater than 3, more preferably equal to orgreater than 5, and particularly preferably equal to or greater than 8.The difference (Ho−Hc) is preferably equal to or less than 40, morepreferably equal to or less than 35, and particularly preferably equalto or less than 30.

When the golf ball 2 that includes the thin cover 8 is hit with adriver, the spin rate is low. When the golf ball 2 is hit with a longiron or a middle iron, the spin rate is also low. The golf ball 2 hasexcellent flight performance. In this respect, the cover 8 has athickness of preferably less than 0.8 mm. The thickness is morepreferably equal to or less than 0.6 mm, even more preferably equal toor less than 0.5 mm, and particularly preferably equal to or less than0.4 mm. In light of controllability upon a shot with a short iron, thethickness is preferably equal to or greater than 0.10 mm andparticularly preferably equal to or greater than 0.15 mm.

For forming the cover 8, known methods such as injection molding,compression molding, and the like can be used. When forming the cover 8,the dimples 14 are formed by pimples formed on the cavity face of amold.

In light of feel at impact, the golf ball 2 has an amount of compressivedeformation of preferably 2.3 mm or greater, more preferably 2.4 mm orgreater, and particularly preferably 2.5 mm or greater. In light ofresilience performance, the amount of compressive deformation ispreferably equal to or less than 3.5 mm, more preferably equal to orless than 3.2 mm, and particularly preferably equal to or less than 3.0mm.

At measurement of the amount of compressive deformation, first, the golfball 2 is placed on a hard plate made of metal. A cylinder made of metalgradually descends toward the golf ball 2. The golf ball 2, squeezedbetween the bottom face of the cylinder and the hard plate, becomesdeformed. A migration distance of the cylinder, starting from the statein which an initial load of 98 N is applied to the golf ball 2 up to thestate in which a final load of 1274 N is applied thereto, is measured asan amount of compressive deformation.

If the cover 8 is laminated directly on the mid layer 6, the cover 8does not firmly adhere to the mid layer 6 due to the difference betweenthe material of the cover 8 and the material of the mid layer 6. Thegolf ball 2 preferably includes an adhesive layer between the mid layer6 and the cover 8. The adhesive layer firmly adheres to the mid layer 6and also to the cover 8. The adhesive layer suppresses separation of thecover 8 from the mid layer 6. As described above, the cover 8 of thegolf ball 2 is thin. When the golf ball 2 is hit by the edge of aclubface, a wrinkle is likely to occur. The adhesive layer suppressesoccurrence of a wrinkle. The golf ball 2 is unlikely to break even bybeing repeatedly hit. In the golf ball 2, loss of energy transfer issmall when the golf ball 2 is hit with a golf club. Thus, the golf ball2 has excellent resilience performance.

The adhesive layer is formed by applying an adhesive to the surface ofthe mid layer 6 and drying the adhesive. The base polymer of theadhesive is a two-component curing type epoxy resin. A preferabletwo-component curing type epoxy resin is obtained by curing a bisphenolA type epoxy resin with a curing agent including a polyamine compound.The bisphenol A type epoxy resin is used for the two-component curingtype epoxy resin, and thus the two-component curing type epoxy resin hasexcellent flexibility, chemical resistance, heat resistance, andtoughness.

The adhesive is obtained by mixing a base material including a bisphenolA type epoxy resin and a solvent with a curing agent including apolyamine compound and a solvent. Examples of the solvents in the basematerial and the curing agent include organic solvents such as xyleneand toluene and water.

Specific examples of the polyamine compound include polyamide amines andmodified products thereof. A polyamide amine has a plurality of aminogroups and one or more amide groups. The amino groups can react withepoxy groups. A polyamide amine is obtained by a condensation reactionof a polymerized fatty acid and a polyamine. A typical polymerized fattyacid is obtained by heating and combining natural fatty acids includinga large amount of unsaturated fatty acids, such as linoleic acid,linolenic acid, and the like, in the presence of a catalyst. Specificexamples of unsaturated fatty acids include tall oil, soybean oil,linseed oil, and fish oil. A hydrogenated polymerized fatty acid havinga dimer content of 90% by weight or greater and a trimer content of 10%by weight or less is preferred. Examples of preferable polyaminesinclude polyethylene diamines, polyoxyalkylene diamines, and derivativesthereof.

The adhesive has a gel fraction of 40% or greater. In the adhesive layerformed from the adhesive having a gel fraction of 40% or greater, avolatile component is unlikely to remain and thus there are few bubblesincluded. The adhesive layer firmly adheres to the mid layer 6 and alsoto the cover 8. In this respect, the gel fraction is more preferablyequal to or greater than 45% and particularly preferably equal to orgreater than 50%.

The adhesive has a gel fraction of 80% or less. The adhesive having agel fraction of 80% or less sufficiently reacts with the base polymer ofthe mid layer 6 and also with the base polymer of the cover 8. Theadhesive layer firmly adheres to the mid layer 6 and also to the cover8. In this respect, the gel fraction is more preferably equal to or lessthan 76% and particularly preferably equal to or less than 70%.

The adhesive layer formed from the adhesive of which the gel fraction isequal to or greater than 40% but equal to or less than 80% exerts aremarkable effect in the golf ball 2 that includes the thin cover 8. Theadhesive layer formed from the adhesive of which the gel fraction isequal to or greater than 40% but equal to or less than 80% exerts aremarkable effect in the golf ball 2 that includes the flexible cover 8.

At measurement of the gel fraction, immediately after the base materialand the curing agent are mixed, the adhesive is applied to a PB-137Tzinc phosphate treated steel plate. The size of the steel plate is “150mm×70 mm”. The thickness of the steel plate is 0.8 mm. The steel plateis kept in the environment of 40° C. for 24 hours to form a coating filmformed from the adhesive. A test piece is obtained from the steel plateand the coating film. The weight of the test piece is measured, and aweight M1 of the coating film is calculated by subtracting the weight ofthe steel plate from the measurement value. The test piece is immersedin acetone and allowed to stand for 24 hours. The test piece is kept inthe environment of 105° C. for one hour. The test piece is cooled to 23°C. The weight of the test piece is measured, and a weight M2 of thecoating film is calculated by subtracting the weight of the steel platefrom the measurement value. A gel fraction G is calculated by thefollowing mathematical formula.

G=(M2/M1)·100

The ratio of the epoxy equivalent of the bisphenol A type epoxy resin tothe amine active hydrogen equivalent of the curing agent in the adhesiveis preferably equal to or greater than 2.0/1.0 but equal to or less than13.0/1.0. In the adhesive in which the ratio is equal to or greater than2.0/1.0, the gel fraction is not too low. Therefore, the adhesive layerfirmly adheres to the mid layer 6 and the cover 8. In this respect, theratio is more preferably equal to or greater than 2.6/1.0 andparticularly preferably equal to or greater than 4.0/1.0. In theadhesive in which the ratio is equal to or less than 13.0/1.0, the gelfraction is not too high. Therefore, the adhesive layer firmly adheresto the mid layer 6 and the cover 8. In this respect, the ratio is morepreferably equal to or less than 12.2/1.0 and particularly preferablyequal to or less than 10.0/1.0.

The amine active hydrogen equivalent of the curing agent is preferablyequal to or greater than 100 g/eq but equal to or less than 800 g/eq. Inthe adhesive in which the equivalent is equal to or greater than 100g/eq, the gel fraction is not too high. Therefore, the adhesive layerfirmly adheres to the mid layer 6 and the cover 8. In this respect, theequivalent is more preferably equal to or greater than 200 g/eq andparticularly preferably equal to or greater than 300 g/eq. In theadhesive in which the equivalent is equal to or less than 800 g/eq, thegel fraction is not too low. Therefore, the adhesive layer firmlyadheres to the mid layer 6 and the cover 8. In this respect, theequivalent is more preferably equal to or less than 600 g/eq andparticularly preferably equal to or less than 500 g/eq.

The adhesive includes water as a volatile component. The term “volatilecomponent” means both water and an organic solvent. The proportion Pw ofwater to the entire volatile component is preferably equal to or greaterthan 90% by weight. In the adhesive in which the proportion Pw is equalto or greater than 90% by weight, the gel fraction is easily controlled.In this respect, the proportion Pw is more preferably equal to orgreater than 95% by weight and particularly preferably equal to orgreater than 99% by weight. The proportion Pw may be 100% by weight. Inlight of the environment, the proportion Po of the organic solvent tothe entire volatile component is preferably equal to or less than 10% byweight, more preferably equal to or less than 5% by weight, andparticularly preferably equal to or less than 1% by weight.

The adhesive layer may include additives such as a coloring agent(typically, titanium dioxide), an antioxidant, a light stabilizer, afluorescent brightener, an ultraviolet absorber, an anti-blocking agent,and the like. The additives may be added to the base material or thecuring agent.

As described above, the adhesive layer is obtained by applying theadhesive to the surface of the mid layer 6. The application can beconducted by a spray gun method, an electrostatic coating method, or adipping method. In light of workability, the application by the spraygun method is preferred. After the application, the solvent isvolatilized to permit a reaction of the bisphenol A type epoxy resinwith the polyamine compound, thereby forming the adhesive layer.

In light of durability of the golf ball 2, the adhesive layer has athickness of preferably 0.001 mm or greater and particularly preferably0.002 mm or greater. The thickness is preferably equal to or less than0.1 mm. The thickness is measured by observing a cross-section of thegolf ball 2 with a microscope. When the mid layer 6 has concavities andconvexities on its surface from surface roughening, the thickness ismeasured at a convex part. The measurement is conducted so as to avoidthe positions of the dimples 14.

The adhesive strength between the mid layer 6 (first layer) and thecover 8 (second layer) is preferably equal to or greater than 20 N. Thegolf ball 2 in which the adhesive strength is equal to or greater than20 N has excellent durability. In this respect, the adhesive strength ismore preferably equal to or greater than 22.0 N and particularlypreferably equal to or greater than 22.3 N.

At measurement of the adhesive strength, a test piece including thefirst layer, the adhesive layer, and the second layer is cut out fromthe golf ball 2. The size of the test piece is “10 mm×50 mm”. At an endof the test piece, the second layer is peeled from the first layer. Thefirst layer is fixed to a first chuck, and the second layer is fixed toa second chuck. The second chuck is moved relative to the first chuck topeel the first layer from the second layer. The force applied at thispeeling is measured. For the measurement, “autograph AG-IS” manufacturedby SHIMADZU CORPORATION is used. The tensile rate is 50 mm/min.

Also for a golf ball that includes a center, an envelope layer formedfrom a resin composition and covering the center, a mid layer formedfrom a resin composition and covering the envelope layer, and a coverformed from a resin composition and covering the mid layer, an adhesivelayer is effective. In the golf ball, an adhesive layer may be presentbetween the envelope layer and the mid layer, and an adhesive layer maybe present between the mid layer and the cover. When the adhesive layeris present between the envelope layer and the mid layer, the envelopelayer is a first layer, and the mid layer is a second layer. When theadhesive layer is present between the mid layer and the cover, the midlayer is a first layer, and the cover is a second layer.

EXAMPLES

The following will show the effects of the present invention by means ofExamples, but the present invention should not be construed in a limitedmanner based on the description of these Examples.

Example 1

A rubber composition (4) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (trade name “BR-730”, manufactured by JSRCorporation), 28 parts by weight of methacrylic acid, 34.8 parts byweight of magnesium oxide, an appropriate amount of barium sulfate, and1.35 parts by weight of dicumyl peroxide. The rubber composition (4) wasplaced into a mold including upper and lower mold halves each having ahemispherical cavity, and heated at 170° C. for 20 minutes to obtain acenter with a diameter of 15 mm.

A rubber composition (9) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (the aforementioned “BR-730”), 33 parts byweight of zinc diacrylate, 5 parts by weight of zinc oxide, anappropriate amount of barium sulfate, 0.33 parts by weight ofbis(pentabromophenyl)disulfide, and 0.90 parts by weight of dicumylperoxide. Half shells were formed from the rubber composition (9). Thecenter was covered with two half shells. The center and the half shellswere placed into a mold including upper and lower mold halves eachhaving a hemispherical cavity, and heated at 150° C. for 20 minutes toobtain a core with a diameter of 40.1 mm. An envelope layer was formedfrom the rubber composition (9). The amount of barium sulfate wasadjusted such that the specific gravity of the envelope layer agreeswith the specific gravity of the center and the weight of a golf ball is45.4 g.

A resin composition (a) was obtained by kneading 50 parts by weight ofan ionomer resin (the aforementioned “Surlyn 8945”) and 50 parts byweight of another ionomer resin (the aforementioned “Himilan AM7329”)with a twin-screw kneading extruder. The core was placed into a moldincluding upper and lower mold halves each having a hemisphericalcavity. The core was covered with the resin composition (a) by injectionmolding to form a mid layer with a thickness of 1.0 mm.

An adhesive including a base material and a curing agent was prepared.The base material is a water-based epoxy composition manufactured bySHINTO PAINT CO., LTD. The base material includes 36 parts by weight ofa bisphenol A type epoxy resin and 64 parts by weight of water. Theepoxy equivalent of the base material is 1405 g/eq. The curing agent isa water-based amine composition manufactured by SHINTO PAINT CO., LTD.The curing agent includes 44 parts by weight of a modified polyamideamine, 50 parts by weight of water, 1 parts by weight of propyleneglycol, and 5 parts by weight of titanium dioxide. The active hydrogenequivalent of the curing agent is 348 g/eq. This adhesive was applied tothe surface of the mid layer with a spray gun, and kept at 23° C. for 12hours to obtain an adhesive layer with a thickness of 0.003 mm.

A resin composition (c) was obtained by kneading 100 parts by weight ofa thermoplastic polyurethane elastomer (trade name “Elastollan NY84 Å”,manufactured by BASF Japan Ltd.) and 4 parts by weight of titaniumdioxide with a twin-screw kneading extruder. Half shells were obtainedfrom the resin composition (c) by compression molding. The sphereconsisting of the core, the mid layer, and the adhesive layer wascovered with two of these half shells. The sphere and the half shellswere placed into a final mold that includes upper and lower mold halveseach having a hemispherical cavity and that has a large number ofpimples on its cavity face. A cover was obtained by compression molding.The thickness of the cover was 0.3 mm. Dimples having a shape that isthe inverted shape of the pimples were formed on the cover. A clearpaint including a two-component curing type polyurethane as a basematerial was applied to this cover to obtain a golf ball of Example 1with a diameter of 42.7 mm. A hardness distribution of the core ofExample 1 is shown in Table 4 below.

Examples 2 to 8 and Comparative Examples 1 to 7

Golf balls of Examples 2 to 8 and Comparative Examples 1 to 7 wereobtained in the same manner as Example 1, except the specifications ofthe center, the envelope layer, the mid layer, and the cover were asshown in Tables 7 to 10 below. The rubber composition of the core isshown in detail in Tables 1 and 2 below. The resin compositions of themid layer and the cover are shown in detail in Table 3 below. A hardnessdistribution of the core is shown in Tables 4 to 6 below. In ComparativeExample 4, after heating at 150° C. for 30 minutes, heating wasconducted at 170° C. for 5 minutes to obtain a center. The golf ballaccording to Comparative Example 7 does not have an envelope layer.

[Shot with Driver (W#1)]

A driver with a titanium head (trade name “SRIXON Z-TX SV-3020) T-65”,manufactured by SRI Sports Limited, shaft hardness: X, loft angle: 8.5°)was attached to a swing machine manufactured by Golf Laboratories, Inc.A golf ball was hit under the condition of a head speed of 50 m/sec. Theball speed and the spin rate immediately after the hit and the distancefrom the launch point to the stop point were measured. The average valueof data obtained by 12 measurements is shown in Tables 7 to 10 below.

[Shot with Middle Iron (I#5)]

A 5-iron (trade name “SRIXON Z-TX”, manufactured by SRI Sports Limited)was attached to the swing machine. A golf ball was hit under thecondition of a head speed of 41 m/sec. The ball speed and the spin rateimmediately after the hit and the distance from the launch point to thestop point were measured. The average value of data obtained by 12measurements is shown in Tables 7 to 10 below.

[Shot with Short Iron (SW)]

A sand wedge (trade name “CG15 Tour ZIP”, manufactured by SRI SportsLimited, loft angle: 58°) was attached to the swing machine. A golf ballwas hit under the condition of a head speed of 21 m/sec, and the spinrate was measured immediately after the hit. The average value of dataobtained by 12 measurements is shown in Tables 7 to 10 below. Inaddition, water was applied to a clubface and a golf ball, and the golfball was hit. The spin rate was measured immediately after the hit. Theaverage value of data obtained by 12 measurements is shown in Tables 7to 10 below.

[Durability Test]

A driver with a titanium head (trade name “XXIO”, manufactured by SRISports Limited, shaft hardness: S, loft angle: 11.0°) was attached to aswing machine manufactured by Golf Laboratories, Inc. A golf ball wasrepeatedly hit under the condition of a head speed of 45 m/sec. Thenumber of hits required to break the golf ball was counted. When thecover was not broken and the core or the mid layer was broken, thebreakage was recognized through deformation of the golf ball or unusualsound at hit of the golf ball. The evaluation was categorized as followson the basis of an index of the average value of data obtained by 12measurements.

A: equal to or greater than 150

B: equal to or greater than 100 but less than 150

C: less than 100

The results are shown in Tables 7 to 10 below. The durability is good inorder of A, B, and C.

[Feel at Impact]

Ten golf players hit golf balls with sand wedges and were asked aboutfeel at impact. The evaluation was categorized as follows on the basisof the number of golf players who answered, “the feel at impact wasexcellent”.

A: 8 or more

B: 6 to 7

C: 4 to 5

D: 3 or less

The results are shown in Tables 7 to 10 below.

TABLE 1 Compositions of Center and Envelope Layer (1) (2) (3) (4) (5)(6) (7) BR-730 100 100 100 100 100 100 100 Zinc diacrylate 15 38Methacrylic acid 22 28 28 28 28 Magnesium oxide 34.8 34.8 34.8 34.8 34.8Zinc oxide 5 5 Barium sulfate * * * * * * * Diphenyl disulfide 0.50 0.50Bis(pentabromophenyl) disulfide Dicumyl peroxide 0.87 1.27 1.30 1.351.38 0.70 0.80 * = Appropriate amount

TABLE 2 Compositions of Center and Envelope Layer (8) (9) (10) (11) (12)(13) BR-730 100 100 100 100 100 100 Zinc diacrylate 33 33 33 43 23 30Methacrylic acid Magnesium oxide Zinc oxide 5 5 5 5 5 5 Bariumsulfate * * * * * * Diphenyl disulfide 0.50 0.50 Bis(pentabromophenyl)disulfide 0.30 0.33 0.35 0.30 Dicumyl peroxide 0.90 0.90 0.90 0.70 0.700.90 * = Appropriate amount

TABLE 3 Compositions of Mid Layer and Cover (a) (b) (c) (d) Surlyn 894550 Himilan AM7329 50 Polyurethane *1 100 Polyurethane *2 100Polyurethane *3 100 Titanium dioxide 4 4 4 Molecular weight of PTMG 15001500 1800 E″ (× 10⁷ Pa) 1.80 3.51 3.24 G″ (× 10⁷ Pa) 1.02 1.42 2.77E″/G″ 1.76 2.47 1.17 Hardness (JIS-C) 94 45 47 47 Hardness (Shore D) 6430 32 32 The details of the polyurethanes in Table 3 are as follows.Polyurethane *1 Thermoplastic polyurethane elastomer Polyol component:polytetramethylene ether glycol Number average molecular weight ofpolyol component: 1500 Polyurethane *2 Thermoplastic polyurethaneelastomer Polyol component: polytetramethylene ether glycol Numberaverage molecular weight of polyol component: 1500 Polyurethane *3 Tradename “Elastollan NY85A”, manufactured by BASF Japan Ltd. Polyolcomponent: polytetramethylene ether glycol Number average molecularweight of polyol component: 1800

TABLE 4 Hardness Distribution of Core Distance from central point Comp.[mm] Example 2 Example 1 Example 3 Example 1 Example 4 0 68.3 67.9 68.467.8 67.9 5.0 68.3 68.0 68.3 68.3 68.0 6.5 68.3 68.9 68.9 68.8 68.9 8.070.4 8.5 70.0 69.2 70.3 69.2 10.0 72.6 70.5 72.1 72.5 70.5 15.0 81.681.1 82.3 82.0 81.1 20.1 84.3 86.0 85.2 86.0 86.0

TABLE 5 Hardness Distribution of Core Distance from central point Comp.[mm] Example 5 Example 6 Example 2 Example 7 0 67.9 69.6 67.9 67.9 5.068.0 70.2 68.0 68.0 6.5 68.9 69.4 68.9 68.9 8.5 69.2 70.0 69.0 69.1 10.070.5 72.9 70.2 70.6 15.0 81.1 80.9 81.5 81.3 19.6 85.8 19.7 86.0 20.186.0 85.8

TABLE 6 Hardness Distribution of Core Distance from central point Comp.Comp. Comp. Comp. Comp. [mm] Example 8 Example 3 Example 4 Example 5Example 6 Example 7 0 67.9 53.0 56.3 62.1 65.0 68.0 5.0 68.0 53.3 56.762.2 67.0 76.0 6.5 68.9 62.4 68.0 8.0 58.5 57.8 8.5 69.2 64.6 69.0 9.060.0 58.0 10.0 70.5 77.7 77.7 73.0 71.0 76.0 15.0 81.1 82.6 82.9 81.078.5 80.0 19.3 83.0 19.9 88.7 89.1 20.1 86.0 84.3 86.0

TABLE 7 Results of Evaluation Ex. 2 Ex. 1 Ex. 3 Comp. Ex. 1 Ex. 4 CenterComposition (3) (4) (5) (4) (4) Crosslinking 170 170 170 170 170temperature [° C.] Crosslinking time [min] 20 20 20 20 20 Diameter [mm]15 15 15 18 15 Envelope Composition (8) (9) (10)  (9) (9) layerCrosslinking 150 150 150 150 150 temperature [° C.] Crosslinking time[min] 20 20 20 20 20 Core Diameter [mm] 40.1 40.1 40.1 40.1 40.1 Volumeratio [%] 82.8 82.8 82.8 82.8 82.8 Central hardness 68.3 67.9 68.4 67.867.9 Ho [JIS-C] Surface hardness 84.3 86.0 85.2 86.0 86.0 He [JIS-C] He− Ho 16.0 18.1 16.8 18.2 18.1 Hardness FIG. 3 FIG. 2 FIG. 4 FIG. 7 FIG.2 distribution H1-Ho 1.7 1.3 1.9 4.7 1.3 Mid Composition (a) (a) (a) (a)(a) layer Hardness Hm 94.0 94.0 94.0 94.0 94.0 [JIS-C] Thickness [mm]1.0 1.0 1.0 1.0 1.0 Cover Composition (c) (c) (c) (c) (d) Hardness Hc47.0 47.0 47.0 47.0 47.0 [JIS-C] Thickness [mm] 0.3 0.3 0.3 0.3 0.3 BallAmount of 2.52 2.61 2.54 2.51 2.61 compressive deformation [mm] W#1 Ballspeed [m/s] 73.6 73.7 73.7 73.5 73.6 Spin [rpm] 2250 2232 2241 2180 2261Flight distance 284.5 284.4 284.7 284.9 284.2 [m] I#5 Spin [rpm] 48244801 4810 4750 4920 Flight distance 182.8 183.0 183.2 183.5 182.5 [m] SWDry spin [rpm] 6578 6550 6564 6480 6540 Wet spin [rpm] 5740 5712 57405650 5600 Durability A A A A A Feel at impact A A A A A

TABLE 8 Results of Evaluation Comp. Ex. 5 Ex. 6 Ex. 2 Ex. 7 CenterComposition (4) (2) (4) (4) Crosslinking 170 170 170 170 temperature [°C.] Crosslinking time 20 20 20 20 [min] Diameter [mm] 15 15 15 15Envelope Composition (9) (8) (9) (9) layer Crosslinking 150 150 150 150temperature [° C.] Crosslinking time 20 20 20 20 [min] Core Diameter[mm] 40.1 40.1 39.1 39.3 Volume ratio [%] 82.1 82.1 76.8 78.0 Centralhardness 67.9 69.6 67.9 67.9 Ho [JIS-C] Surface hardness 86.0 85.8 85.886.0 He [JIS-C] He − Ho 18.1 16.2 17.9 18.1 Hardness FIG. 2 FIG. 5 FIG.8 FIG. 6 distribution H1-Ho 1.3 0.4 1.1 1.2 Mid Composition (a) (a) (a)(a) layer Hardness Hm 94.0 94.0 94.0 94.0 [JIS-C] Thickness [mm] 1.0 1.01.0 1.0 Cover Composition (b) (c) (c) (c) Hardness Hc 45.0 47.0 47.047.0 [JIS-C] Thickness [mm] 0.3 0.3 0.8 0.7 Ball Amount of 2.60 2.552.55 2.57 compressive deformation [mm] W#1 Ball speed [m/s] 73.7 73.773.6 73.6 Spin [rpm] 2272 2200 2400 2362 Flight distance 283.8 285.0282.5 283.0 [m] I#5 Spin [rpm] 4960 4800 5051 5010 Flight distance 181.5183.0 181.5 181.3 [m] SW Dry spin [rpm] 6644 6500 6810 6770 Wet spin[rpm] 5830 5680 5986 5950 Durability A A A A Feel at impact A A A A

TABLE 9 Results of Evaluation Comp. Comp. Ex. 8 Ex. 3 Ex. 4 CenterComposition (4)  (6)  (6) Crosslinking 170 170 150 170 temperature [°C.] Crosslinking time 20 15 30 5 [min] Diameter [mm] 15 18 18 EnvelopeComposition (9) (11) (11) layer Crosslinking 150 170 170 temperature [°C.] Crosslinking time 20 20 20 [min] Core Diameter [mm] 40.1 39.7 39.7Volume ratio [%] 82.8 80.4 80.4 Central hardness 67.9 53.0 56.3 Ho[JIS-C] Surface hardness 86.0 88.7 89.1 He [JIS-C] He − Ho 18.1 35.732.8 Hardness FIG. 2 FIG. 9 FIG. 10 distribution H1-Ho 1.3 24.7 21.4 MidComposition (a) (a) (a) layer Hardness Hm 94.0 94.0 94.0 [JIS-C]Thickness [mm] 0.8 1.0 1.0 Cover Composition (c) (d) (c) Hardness Hc47.0 47.0 47.0 [JIS-C] Thickness [mm] 0.5 0.5 0.5 Ball Amount of 2.602.54 2.53 compressive deformation [mm] W#1 Ball speed [m/s] 73.7 73.273.3 Spin [rpm] 2300 2062 2142 Flight distance 283.5 285.8 284.5 [m] I#5Spin [rpm] 4900 4720 4910 Flight distance 182.5 183.8 181.9 [m] SW Dryspin [rpm] 6650 6420 6450 Wet spin [rpm] 5820 5590 5470 Durability B C CFeel at impact A B B

TABLE 10 Results of Evaluation Comp. Comp. Comp. Ex. 5 Ex. 6 Ex. 7Center Composition (1) (12) (7) Crosslinking 150 170 170 temperature [°C.] Crosslinking time 15 15 15 [min] Diameter [mm] 15 15 40.1 EnvelopeComposition (9) (13) — layer Crosslinking 150 150 — temperature [° C.]Crosslinking time 20 20 — [min] Core Diameter [mm] 40.1 38.5 40.1 Volumeratio [%] 82.8 73.3 82.8 Central hardness 62.1 65.0 68.0 Ho [JIS-C]Surface hardness 84.3 83.0 86.0 He [JIS-C] He − Ho 22.2 18.0 18.0Hardness FIG. 11 FIG. 12 FIG. 13 distribution H1-Ho 2.5 4.0 — MidComposition (a) (a) (a) layer Hardness Hm 94.0 94.0 94.0 [JIS-C]Thickness [mm] 1.0 1.0 1.0 Cover Composition (c) (c) (c) Hardness Hc47.0 47.0 47.0 [JIS-C] Thickness [mm] 0.3 0.3 0.3 Ball Amount of 2.632.40 2.48 compressive deformation [mm] W#1 Ball speed [m/s] 73.5 73.873.9 Spin [rpm] 2230 2680 2125 Flight distance 284.0 258.9 283.4 [m] I#5Spin [rpm] 4820 4980 5110 Flight distance 182.8 181.7 181.0 [m] SW Dryspin [rpm] 6500 6710 6560 Wet spin [rpm] 5750 4370 5600 Durability C A AFeel at impact B B A

As shown in Tables 7 to 10, the golf balls according to Examples areexcellent in various performance characteristics. From the results ofevaluation, advantages of the present invention are clear.

The golf ball according to the present invention can be used for playinggolf on golf courses and practicing at driving ranges. This is anexample.

The above descriptions are merely for illustrative examples, and variousmodifications can be made without departing from the principles of thepresent invention.

1. A golf ball comprising a core, a mid layer positioned outside thecore, and a cover positioned outside the mid layer, wherein the corecomprises a center and an envelope layer positioned outside the center,a ratio of a volume of the core to a volume of a phantom sphere of thegolf ball is equal to or greater than 78%, a difference (He−Ho) betweena JIS-C hardness He at a surface of the core and a JIS-C hardness Ho ata central point of the core is equal to or greater than 15 but equal toor less than 40, and a difference (H1−Ho) between a JIS-C hardness H1 ata point P1 that is located radially outward of a boundary between thecenter and the envelope layer and whose distance from the boundary is 1mm and the JIS-C hardness Ho at the central point of the core is equalto or greater than 0 but equal to or less than
 2. 2. The golf ballaccording to claim 1, wherein a JIS-C hardness Hc of the cover is lessthan
 65. 3. The golf ball according to claim 1, wherein a thickness ofthe cover is less than 0.8 mm.
 4. The golf ball according to claim 1,wherein a JIS-C hardness Hm of the mid layer is equal to or greater than90.
 5. The golf ball according to claim 1, wherein a thickness of themid layer is equal to or less than 1.5 mm.
 6. The golf ball according toclaim 1, wherein a JIS-C hardness Hc of the cover is less than the JIS-Chardness Ho at the central point of the core.
 7. The golf ball accordingto claim 1, wherein a JIS-C hardness Hm of the mid layer is greater thanthe JIS-C hardness He at the surface of the core.
 8. The golf ballaccording to claim 1, wherein the cover is formed from a resincomposition, a shear loss elastic modulus G″ of the resin composition,which is measured under conditions of a vibration frequency of 10 Hz anda temperature of 0° C., is equal to or less than 1.95×10⁷ Pa, and aratio (E″/G″) of a tensile loss elastic modulus E″ of the resincomposition, which is measured under conditions of a vibration frequencyof 10 Hz and a temperature of 0° C., to the shear loss elastic modulusG″ is equal to or greater than 1.76.
 9. The golf ball according to claim8, wherein a principal component of a base material of the resincomposition is a thermoplastic polyurethane, and a polyol component ofthe thermoplastic polyurethane is polytetramethylene ether glycol havinga number average molecular weight of 1500 or less.
 10. The golf ballaccording to claim 1, wherein the JIS-C hardness Ho at the central pointof the core is equal to or greater than 40 but equal to or less than 80.11. The golf ball according to claim 1, wherein a diameter of the centeris equal to or greater than 10 mm but equal to or less than 20 mm. 12.The golf ball according to claim 1, wherein the envelope layer isobtained by a rubber composition being crosslinked.
 13. The golf ballaccording to claim 1, wherein a JIS-C hardness at a surface of theenvelope layer is equal to or greater than 75 but equal to or less than95.
 14. The golf ball according to claim 1, wherein a thickness of theenvelope layer is equal to or greater than 8 mm but equal to or lessthan 18 mm.
 15. The golf ball according to claim 1, wherein a difference(Ho−Hc) between a JIS-C hardness Hc of the cover and the JIS-C hardnessHo at the central point of the core is equal to or greater than 3 butequal to or less than 40.